1. Field of the Invention
This invention relates to a gamma correction circuit.
2. Description of the Related Art
In a liquid crystal display device, for example, the relation between an applied voltage and a transmittance of a liquid crystal is non-linear as shown in FIG. 5. In order to obtain a linear relation between a video signal and the transmittance of the liquid crystal, it is necessary to make a correction by using a circuit having a characteristic as shown in FIG. 6. FIG. 7 shows an example of a gamma correction circuit for a liquid crystal display device, which is intended for the above-mentioned correction. The transistors Q21 and Q22, resistors R21 and R22, and constant-current circuits I21 and I22 constitute an amplifier, and a voltage inputted to the base of the transistor Q21 is amplified and outputted from the collector of the transistor Q22. The amplification factor of this amplifier is substantially R21/R22. (Hereafter, the resistance values of the resistors are expressed by their reference numerals.)
When an input voltage VIN increases and an resulting output voltage VO exceeds a base voltage VA of the transistor Q25, the transistor Q23 turns on. Consequently, the resistor R23 is connected in parallel with the resistor R21, so that the value of a load resistor of the transistor Q22 becomes R21.multidot.R23/(R21+R23), with the result that the amplification factor of the amplifier becomes R21.multidot.R23/{(R21+R23).multidot.R22}. That is to say, when the output voltage Vo exceeds the voltage VA, the amplification factor is compressed by R23/(R21+R23).
As the output voltage VO increases further, the current flowing through the resistor R23 increases, causing the current flowing through the resistor R24 to increase, and therefore, the base voltage of the transistor Q24 rises. When this base voltage becomes larger than the voltage VB, the collector current of the transistor Q26 decreases, so that the base current of the transistor Q23 decreases. As a result, the collector current of the transistor Q23 decreases, and the base voltage of the transistor Q24 falls. In this way, the base voltage of the transistor Q24 is stabilized at the voltage VB and remains unchanged even though the output voltage VO exceeds the voltage VB. In other words, the collector current of the transistor Q23 is made constant and the resistor R23 is isolated electrically from the amplifier. As a result, the amplification factor of the amplifier returns to the original value R21/R22.
FIG. 8 shows an input/output characteristic of this prior gamma correction circuit whose amplification factor changes with the value of the input voltage.
In such a prior gamma correction circuit, since the circuit comprising the resistor R23 and the transistor Q23 is connected to the output of the amplifier, that is, to the collector of the transistor Q22, a frequency characteristic of the gamma correction circuit is deteriorated by its parasitic capacitance compared with a case in which its load is only the resistor R21. Another problem is that the phase characteristic is changed since the value of the load is changed with the change of the level of the input signal, accordingly with the change of the output signal.